In recent years, advanced microelectronic devices are fabricated with very large scale integration (VLSI) or ultra large scale integration (ULSI) techniques such that extremely complex electrical circuits can be fabricated onto a very small chip. The large reduction in size of the microelectronic devices requires the development of new design and manufacturing technologies to accomplish the miniaturization of such semiconductor devices. One of the critical fabrication steps for the microelectronic devices is the photolithographic process in which a pattern of the circuits in a microscopic scale is transferred from a photomask onto a wafer surface such that the circuits are reproduced on the wafer.
A typical photolithographic process utilizes a step-and-repeat process to gradually transfer a mask pattern to a chip implementation on a microscopic scale. The process involves many individual steps of reductions wherein errors may be introduced into the final mask. For instance, in such a micro-lithography process, problems exist in the accurate measurement of overlay which is an indication of the degree of misalignment between successive layers of patterns on a semiconductor wafer surface and of the alignment of a mask/reticle pattern for printing such layers to preceding layers. In conducting lithography on such a microscopic scale, the capability of making an accurate measurement of overlay is a critical requirement of the process.
In a conventional method for measuring overlay and for aligning the wafers, a global alignment method and global alignment marks are frequently utilized wherein alignment marks are patterned by an independent process layer and then all the other device process layers are aligned to the alignment marks. For the device layers, the degree of overlay shift (or error) can be measured to by specifically designed measurement patterns from both successive and preceding layers formed by scribe lines for checking the overlay. For instance, the test targets can be laid out in the peripheral regions on a wafer surface. The measurements are then conducted to verify the accuracy of the circuit lay out by comparing shifts in the box center lines to a process average. With the conventional techniques, an accurate wafer dimensional quality determination cannot be made until box-in-box targets are made on an appropriate number of cells within a circuit. Furthermore, in the conventional measurement techniques, by using a stepper machine, a focal plane of the stepper is determined manually by technicians by visually reading exposed 0.6 .mu.m focus matrix dots.
In a stepper machine, an excimer laser such as one formed by krypton/fluorine gas mixture is frequently used for triggering a 240 nm exposure light in the stepper. The KrF excimer laser is generated in a laser chamber that is filled with both the krypton gas and the fluorine gas. The laser chamber contains the gas mixture under a sufficient pressure. Laser energy is generated when electrical energy is discharged into the gas mixture in the chamber. A blower fan inside the chamber cavity circulates the laser gas mixture between each electrical discharge. The chamber is normally constructed of a specific refractory material such that the lifetime of the laser gas may be extended. A unique pre-ionization process is further utilized to allow the process to be operated at significantly reduced voltages.
A conventional excimer laser chamber for generating a laser for the stepper and its related fluorine gas treatment apparatus is shown in FIG. 1. The laser generating apparatus 10 consists of an excimer laser chamber 12 which is used for generating a laser, a fluorine trap 14 for removing fluorine from the exhaust gas from the laser chamber 12, a transparent window 16 mounted in the conduit 18 to allow visual inspection of the conduit interior, a fluorine gas sensor 20 and a gas evacuation device 22. The fluorine trap 14 is part of a gas control module (not shown) that handles all gas functions, including the removal of fluorine from the exhaust gas by the fluorine trap 14 before it exits into the atmosphere. The gas module also regulates the flow of nitrogen to various components and subsystems. The transparent window 16 allows visual inspection of the status of the fluorine trap. A laser alignment module (not shown) is further provided which emits a visible laser and provides a means for making alignments between the ultra violet laser and the stepper. The guide laser light is superimposed onto the UV beam path with a mirror mounted on the output coupler assembly.
In the conventional excimer laser chamber shown in FIG. 1, a gas mixture that contains 0.9.about.1.0% fluorine and 1.2.about.1.3% krypton in neon is normally employed. The volume of the gas mixture utilized is approximately 30 liter-ATM per fill at a delivery pressure of 440.about.480 kPa. Since fluorine is the most reactive element and one of the most potent oxidizers, inhalation of and skin contact with even 1% fluorine can be hazardous to human. The exhaust from the excimer laser chamber 12 must therefore be treated with an on-board physical capture device, i.e., a fluorine trap 14. In a conventional semiconductor fabrication process, the fluorine trap 14 is normally replaced after 250 laser fill cycles. It is a costly process since not only it requires significant down time of the laser chamber for the replacement, but also causes unnecessary waste of the costly fluorine trap since frequently only 60.about.70% of the trap capacity is consumed after 250 cycles. While window 16 is also used as an indicator, i.e., when it turns dark for the need of fluorine trap replacement, the conventional trap replacement procedure results in unnecessary down time for the process chamber and a decrease in chip yield.
It is therefore an object of the present invention to provide an apparatus for trapping a toxic gas from a laser chamber that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for trapping a toxic gas such as fluorine from a laser chamber that can be carried out on a minimal cost basis.
It is a further object of the present invention to provide an apparatus for trapping a toxic gas such as fluorine from a laser chamber that utilizes a double-stack fluorine trap.
It is another further object of the present invention to provide an apparatus for trapping a fluorine gas wherein two fluorine traps are connected in series with a fluorine sensor connected thereinbetween.
It is still another object of the present invention to provide an apparatus for trapping a fluorine gas wherein two fluorine traps are connected in series and the downstream trap is used to replace a consumed upstream trap.
It is yet another object of the present invention to provide a method for trapping a toxic gas by providing two toxic gas traps connected in series with a toxic gas sensor thereinbetween.
It is still another further object of the present invention to provide a method for trapping a fluorine gas by utilizing two fluorine gas traps connected in series such that when the first trap is fully consumed, it is replaced by the second trap.
It is yet another further object of the present invention to provide an apparatus for trapping fluorine gas from a laser chamber exhaust by providing two fluorine traps connected in series, a fluorine sensor thereinbetween and a gas pump for withdrawing the exhaust gas from the laser chamber.